Data eyeglasses are a special form of a head-mounted display. One popular form of head-mounted display uses screens that are worn in front of the eyes and presents the user with computer-generated images or images received by cameras. Such head-mounted displays are frequently voluminous and permit no direct perception of the surroundings. Just recently head-mounted displays were developed that can present the user with an image received by a camera or a computer-generated image without impeding direct perception of the surroundings. Such head-mounted displays, which shall hereinafter be referred to as data eyeglasses, permit the use of this technology in everyday life.
Typical data eyeglasses, as described for example in DE 10 2011 007 812 A1 and US 2012/0002294 A1, comprise an image generator to produce an image, and an eyeglass lens to which an imaging beam path proceeding from the image generator is incoupled. By means of an outcoupling region present in the eyeglass lens, the imaging beam path is outcoupled in the direction of the eye of the data eyeglasses wearer. In addition, an imaging optics is present, which causes the imaging beam path to generate a virtual image that can be perceived by the user. Here the outcoupling of the imaging beam path in the direction of the eye occurs by means of a beam-splitting outcoupling region in such a way that the virtual image is superimposed over the image of the surroundings, so that the user perceives the virtual image in addition to the surroundings.
U.S. Pat. No. 5,886,822 describes data eyeglasses with eyeglass lenses in which an imaging beam path can be incoupled to the edge. By means of an outcoupling element, the imaging beam path is outcoupled in the direction of the eye. Further, the data eyeglasses comprise a display and a cone that can be displaced relative to an input end of a gradient-index fiber arranged between the cone and the edge of the eyeglass lens.
US 2013/0314793 A1 describes data eyeglasses with a waveguide, which can be configured as an eyeglass lens, wherein an image generator is arranged on the edge of the waveguide, whose image is incoupled by means of an imaging optics via the edge of the waveguide into the latter. By means of an outcoupling device, the imaging beam path is outcoupled from the waveguide so as to generate a virtual image. Here the outcoupling device has various focusing elements that can optionally be switched on and off. Depending on which focusing element is switched on, the virtual image appears at a different image distance.
So as to be able to present the user of the data eyeglasses with a comfortable and ergonomic virtual image, the imaging optics has a large magnification (typical greater than 100) and is configured so that by means of the virtual image, the observer is given the impression that the scene represented in the virtual image is at a distance of a few meters in front of the eye.
The distance at which the virtual image is perceived by the observer, however, is very sensitive with respect to position and thickness tolerances of the optically effective surfaces. Factors that can influence this distance include manufacturing tolerances, ecological and weather influences based on various operating conditions (air temperature, relative humidity, volume change due to heating, for instance from solar radiation and the like). For this reason, there can be disturbing variations of the distance, in which the virtual image is perceived.